The present invention relates to a semiconductor pressure sensor, and more particularly to an improved semiconductor pressure sensor having higher sensitivity and good temperature characteristics as compared to conventional sensors and capable of being fabricated at a low cost.
Referring to FIG. 1, there is shown a typical arrangement of a known semiconductor pressure sensor of absolute pressure type. The pressure sensor is provided with a hollow shell member 1 comprising a rigid mount plate 1A and a cap 1B fixed to the mount plate 1A, wherein a semiconductor chip 2 of diaphragm type which forms the body portion of the sensor and a semiconductor (e.g. silicon) base member 3 for supporting the chip 2 are accommodated within the shell 1. As well known, the chip 2 is configured by scooping the central portion of a semiconductor thin plate to form a diaphragm portion having a thin thickness, thereafter selectively doping impurities into the diaphragm portion to form a resistor constituting a strain gauge. A lead wire 4 connected to the resistor within the chip 2 is connected to external leads 5 inserted into the shell 1.
In order to apply mechanical displacement to the diaphragm portion of the chip 2 the base member 3 is provided with a pressure inlet bore 3a opening toward the diaphragm portion through the base member 3 supporting the chip 2 and the pressure inlet bore 3a communicates with a bore 1a provided through the mount plate 1A of the shell 1.
The base member 3 serves as not only a supporting member for supporting the chip 2, but also a thermal buffering member for shielding undesired thermal influence externally applied at a time of production of the sensor and/or using thereof. Accordingly, the base member 3 is formed of silicon having the same coefficient of thermal expansion as that of constituent material of the chip 2.
Outside the mount plate 1A of the shell 1, a pressure inlet tube 6 communicating with the bore 1a is fixedly attached. Thus, gas (air) passing through the pressure inlet tube 6 causes mechanical displacement in the diaphragm portion of the chip 2.
The mount plate 1A is further provided with lead leading-out holes for inserting the external lead 5. The mount plate 1A is configured so that a seal ring 7 fitted into each of the lead leading-out holes does not allow air outside the shell 1 to enter thereinto.
The connections between the chip 2 and the base member 3 and between the base member 3 and the mount plate 1A are carried out by means of a solder of Pb-Sn family, respectively. The chip 2 is securely mounted on the mount plate 1A through the base member 3. The mount plate 1A is made of 42% Ni-Fe alloy which will be called "NSD" hereinafter.
When designing such known semiconductor pressure sensors, it is preferable that the height of the base member 3 is as large as possible, while the lateral cross section thereof is as small as possible, in order to minimize thermal variations of characteristics and mechanical distortion applied to the chip. However, if the lateral cross section is small, the diameter of the chip 2 (i.e. the diameter of the diaphragm portion of the chip 2) becomes small. This makes it impossible to sense the pressure with high sensitivity, leading to inconsistent result that high performance in regard to the sensitivity of the sensor cannot be expected.
For this reason, in conventional semiconductor pressure sensors, the base member 3 has been designed so that the cross section of the base member 3 is small and the height of the base member 3 is large in order to meet the requirement of minimization in the above-mentioned undesired thermal influence and mechanical distortion, in addition to this, the chip has been formed so that the diaphragm portion is as thin as possible in order to improve the sensitivity of the sensor. Further, in order that large residual stress and thermal distortion are not produced between the base member 3 having small coefficient of thermal expansion and the mount plate 1A having large coefficient thereof at producing step of the sensor, and that the heat applied to the chip 2 is as small as possible, a soft solder of Pb-Sn family having a low melting point has been used as a bonding material.
The drawbacks with the above-mentioned semiconductor pressure sensors are as follows.
First, the fraction defective is large because it is necessary to make the diaphragm portion of the chip 2 extremely thin, resulting in low yield in the step for production of the chip and high cost in the production thereof.
Second, since the diaphragm of the chip 2 cannot have large diameter, it is impossible to fabricate a high sensitive sensor.
Third, since the pressure inlet bore 3a provided through the base member 3 is formed by an ultrasonic machining which generally takes long time, the larger the height of the base member 3 is, the longer the time required for machining is, resulting in high production cost of the base member 3.
Fourth, since the adhesion area of the base member 3 with respect to the mount plate 1A is still relatively large, thermal distortion produced between the base member 3 and the mount plate 1A due to the heat externally conducting into the shell 1 is considerably large, which is not negligible.
Fifth, since the base member 3 and the mount plate 1A are joined using a solder having a low melting point so that thermal distortion produced therebetween becomes small, the sensor cannot be used under environment of relatively high temperature or under environment where temperature greately changes. Further, as stated above, since the adhesion area of the base member 3 with respect to the mount plate 1A is still large, temperature changes outside the shell have large influence on the chip 2 through the base member 3, resulting in a sensor having bad temperature characteristics.
Sixth, since the height of the base member 3 is large, the height of the inner lead is also large. As a result, there occurs vibration of the lead at a time of bonding, thereby to narrow a range of conditions for optimum wire bonding, thus making it difficult to maintain quality of the bonding.
Seventh, when the base member 3 is joined to the mount plate 1A and then lead wire connection is implemented, thereafter the cap 1B is fixed to the mount plate 1A, if the adhesion area of the base member 3 with respect to the mount plate 1A is large, there occurs large distortion in the base member 3 after the cap 1B has been mounted, resulting in undesired effect on the sensitivity of the sensor, etc. It is to be noted that in the conventional sensors of this kind, the sensitivity of the sensor after the cap is mounted generally lowers as compared to that before the cap is mounted.
In the prior art, in order to lessen inconveniences caused by drawbacks as stated in the above-mentioned fourth and fifth items, as shown in FIGS. 2(a) and 2(b), there have been used improved base members 3A and 3B in which recesses are provided in the radial direction. However, since the adhesion area between these base members 3A and 3B and the mount plate 1A is still large, it is difficult to sufficiently solve fourth and fifth drawbacks and other drawbacks remain as they are.
Further, another attempt has been made to mount the base member on the mount plate through a pipe member. Although inconvenience caused by the fourth and fifth drawbacks can be lessened, no particular consideration is given to the mounting between the pipe member and the base member or the mount plate, resulting in difficulty in sufficiently eliminating thermal distortion, etc.